1. Field of the Invention
The present invention relates to a cosmetic composition for skin comprising urea which is useful for preventing rough dry skin and maintaining moisturization. The cosmetic composition for skin of the present invention is characterized by the capability of suppressing odor from ammonia produced by urea during storage.
2. Description of the Background Art
Significance of a natural moisturizing factor (NMF) which is present in keratinous cells and a sebum cutaneum membrane which covers the skin in naturally maintaining the skin has conventionally been known. If these do not function properly, the moisture content of the skin decreases, resulting in dry and rough skin.
For this reason, various additives which supplement the above-mentioned functions, for example, polyhydric alcohols such as glycerol and propylene glycol, saccharides such as sorbitol and maltitol, amino acids, macromolecule compounds such as hyaluronic compounds and chondroitin acid, and lactic acid bacterium culture supernatant liquid, have conventionally been added to cosmetic compositions for skin.
Urea is known to exhibit various physiological functions and has been used very often in cosmetic compositions. Moisture retention activity of keratin, cell activation function, antibacterial action, antipruritic function, and the like are given as the functions of urea. Urea is thus added to cosmetic compositions and drug compositions with the object of preventing skin chapping and retaining moisture.
However, urea in the compositions decomposes into ammonia and carbon dioxide during storage, increasing the pH and generating an unacceptable ammonia odor.
Various methods have been proposed to solve this problem. For example, Japanese Patent Application Laid-open No. 109487/1977 discloses that the addition of allantoin in an amount of 1 wt % or more for the amount of urea stabilizes urea and can suppress ammonia odor. The technologies which attain the same effect include, for example, a method of adding a buffer to urea to adjust the pH to 6-9 and further adding an ammonium salt, allantoin, an allantoin derivative, or uric acid (Japanese Patent Application Laid-open No. 48441/1976), a method of adding a basic amino acid to urea (Japanese Patent Application Laid-open No. 30509/1986), a method of adding taurine to urea (Japanese Patent Publication No. 81567/1992), a method of adding a neutral amino acid, acidic amino acid, or alkaline salt of an acidic amino acid (Japanese Patent Publication No. 56002/1992), and a method of adding lecitin to urea (Japanese Patent Publication No. 31541/1993, Japanese Patent Application Laid-open No. 364104/1992).
These methods are not always satisfactory in stabilizing urea or in suppressing ammonia odor due to generation of ammonia over time.
Therefore, an object of the present invention is to provide a cosmetic composition comprising urea useful for preventing skin chapping and retaining skin moisture, which does not exhibit a pH increase and generation of an ammonia odor during storage for a long period of time.
The present inventors have conducted extensive studies to obtain a composition incorporating urea in a stable manner. As a result, the inventors have found that the capability of a composition to incorporate urea in a stable manner depends not only on the pH, but also largely on a buffer constituting the system of a composition such as a cosmetic composition. The inventors have further found that a cosmetic composition in which the above-mentioned problems are solved can be obtained by appropriately selecting and using a specific buffer and that, if allantoin is added, a cosmetic composition in which decomposition of urea is suppressed even more can be obtained. These findings have led to the completion of the present invention.
The above object is attained in the present invention by the provision of a cosmetic composition for skin use which comprises (i) urea and (ii) a buffer comprising disodium hydrogenphosphate and citric acid or a buffer comprising potassium dihydrogenphosphate and disodium hydrogenphosphate, or both.
In a preferred embodiment of the present invention, the above cosmetic composition for skin use further comprises allantoin.
Other objects, features and advantages of the invention will hereinafter become more readily apparent from the following description.
Urea is the major NMF component in the cosmetic composition for skin use of the present invention and is incorporated preferably in an amount up to about 5 w/w % (hereinafter indicated as xe2x80x9c%xe2x80x9d).
The other component which is a buffer comprising disodium hydrogenphosphate and citric acid or a buffer comprising potassium dihydrogenphosphate and disodium hydrogenphosphate (hereinafter called xe2x80x9cspecific bufferxe2x80x9d) is incorporated with urea to improve stability of the composition over time. It is possible to use either the disodium hydrogenphosphate-citric acid buffer or the potassium dihydrogenphosphate-disodium hydrogenphosphate buffer, or a combination of both buffers. The amount of the specific buffer to be added should be in the range from 10 to 70%, preferably from 20 to 50%, of the amount of urea. If the amount of the specific buffer is more than 50%, the salt concentration is so high that the resulting cosmetic composition may irritate the skin. In addition, when the composition with a high content of specific buffer is in the form of emulsion such as a milky lotion, the emulsion tends to break.
Use of a buffer to stabilize the pH of cosmetic compositions is widely known. Incorporation of a buffer in a cosmetic composition comprising urea is also known. However, the remarkable stabilizing effect of the above specific buffers as compared with other conventional buffers has not been known in the art. The effect has been found for the first time by the inventors of the present invention. Specifically, other buffers which are commonly used for cosmetic compositions such as a citric acid-sodium hydroxide buffer or a PCA-sodium PCA buffer cannot sufficiently stabilize pH or suppress odor.
The urea-containing cosmetic composition for skin use of the present invention in which a specific buffer is used in combination with urea exhibits an excellent pH stabilizing effect and a superb ammonia odor suppressing effect. However, even a slight ammonia odor may significantly affect the product quality of some cosmetic compositions. Such an odor must be completely suppressed.
In such a case, the use of allantoin can more effectively suppress the ammonia odor. However, of the two specific buffers mentioned above only the disodium hydrogenphosphate-citric acid buffer can suppress the ammonia odor when used in combination with the allantoin. No appreciable effect can be achieved when allantoin is used in combination with another specific buffer, the potassium dihydrogenphosphate-disodium hydrogenphosphate buffer.
The other buffers may or may not suppress the generation of ammonia when used together with allantoin. For example, the combined use of allantoin with a PCA-sodium PCA buffer is not only ineffective in suppressing the ammonia odor, but rather accelerates generation of ammonia odor. In addition, the combined use of allantoin does not significantly affect the capability of a buffer to stabilize pH, nor is the independent use of allantoin effective for suppressing the ammonia odor.
The ammonia odor suppressing effect achieved by the combined use of allantoin is thus not always due to the pH stabilization effect. Although the reason for this effect is not necessarily clear, a synergistic effect of the disodium hydrogenphosphate-citric acid buffer and allantoin is a possible reason.
Although there are no specific limitations to the amount of allantoin, the use of 1% or more of the amount of urea is sufficient for suppressing the ammonia odor, when the allantoin is used in combination with the specific buffer of the present invention.
The amount of the disodium hydrogenphosphate-citric acid buffer used in combination with allantoin is preferably in the range from about 10 to 70%, and more preferably from about 20 to 50%, of the amount of urea.
The initial pH of the urea-containing cosmetic composition for skin use of the present invention prepared by formulating the above-mentioned components is preferably from pH 4.5 to pH 6.5.
The urea-containing cosmetic composition for skin use of the present invention can be prepared by homogeneously mixing the above-described components. The composition can be made into various forms such as a liquid preparation, milky lotion, cream, gel preparation, and the like. Various additives such as, for example, whey (2), sodium hyaluronate, and lamella structure lipid, can optionally be incorporated depending on the form of the preparation inasmuch as the effect of the present invention is not adversely affected.
The urea-containing cosmetic composition for skin use of the present invention can prevent rough dry skin due to urea and can provide a moisturizing effect. At the same time, the cosmetic composition can suppress generation of an ammonia odor originating from urea during storage, particularly, under high temperature conditions. The present invention has thus increased the commercial value of the urea-containing cosmetic composition for skin use.
Other features of the invention will become apparent in the course of the following description of the exemplary embodiments which are given for illustration of the invention and are not intended to be limiting thereof.